The present invention relates to the detection of surface flaws in manufactured articles and more particularly, to the detection of surface flaws in manufactured articles such as can bodies. Even though prior art has provided a number of inductive sensors, such as reluctance, eddy:current, Hall effect, and bridge type using a pair of inductors, the known art is still dependent on the impedance variation of an AC excited uniaxial inductor in proximity to either a ferrous or non-ferrous target. Many types of conventional inductive sensors have been adapted to the detection of dents in the sidewall of a container, such as the Wu et al. U.S. Pat. No. 5,130,653 which utilizes two spool type eddy current sensors connected in a differential bridge circuit. These two spools are coaxially mounted in an axial guide path for the inspected cans. Although this was a step in the right direction for higher line speeds, the spool type sensors have an unfavorable signal/noise ratio because: ##EQU1## In the Wu et al. patent at any given inspection interval each spool type detector coil is receiving an eddy current signal representing a band-like portion encircling the circumference of the can, of which a modest dent (1/8") makes up only a small portion of the total eddy current value being scanned at any given instant. To extract the desired dent signal from this band-like eddy current signal, of course a very high gain pre-amplifier and filter is required, with it's attendant thermal drift, and meticulous balancing procedure. The very careful tweaking adjustments are disclosed in col. 12, line 1, and col. 11, line 1, of the cited patent. The detection system of the present invention utilizes two groups of eight, each circularly arranged rotating magnetic field sensors, to scan the sidewalls of the inspected containers at sixteen longitudinal positions around the cans as they traverse the axial path. In the present sensing apparatus the total sensor area/flaw area ratio is more favorable being divided.
The present sensor is inherently-self balanced, which completely eliminates manual tweaking adjustments.
Also in container manufacture the flange portion of the container must be inspected for cracks and dents before the end is sealed in place. The Wright U.S. Pat. No. 4,029,958 discloses a flange flaw detection apparatus in which the container flange is rapidly rotated near a permanent magnet coupling flux to a pick-up coil.
Dents or cracks in the rotating flange thus produce a changing field coupled to the pick-up coil, generating a flaw signal, the resultant amplitude thus depending on the rotational speed of the container. The sensing frequency is directly related to the rpm of the can (less than 50 hz). Since frequency is a factor of induction, this method is not as sensitive as an oscillator driven unit, and the mechanical rotating equipment is a disadvantage. The Hinds et al. U.S. Pat. No. 4,002,966 discloses another apparatus for detecting flaws in the flange of a container, in which the container flange is rotated near an ac driven field proximity sensor, and the induced eddy current signal is compared to a reference value to extract the flaw signal.